The present invention relates to semiconductor laser devices for use as light sources for pickups in optical disk apparatus or light sources necessary for, for example, other electronic devices and information processors. The present invention particularly relates to semiconductor laser devices having two or more emission wavelengths in red and infrared light regions, for example.
High-capacity digital video disks (DVDs) capable of high-density recording and DVD apparatus for playing back such DVDs are currently on the market and have received attention as products expected to further grow in demand. Since these DVDs are high-density recording media, AlGaInP-based semiconductor lasers having an emission wavelength of 650 nm are used as laser light sources for recording and playback of the DVDs. Accordingly, optical pickups in conventional DVD apparatus could not play back compact discs (CDs) and MiniDiscs (MDs) that are played back using AlGaAs-based semiconductor lasers having an emission wavelength of 780 nm.
To solve this problem, an optical pickup formed by incorporating laser chips in separate packages, specifically, an optical pickup provided with an AlGaInP-based semiconductor laser having an emission wavelength in a 650-nm region and an AlGaAs-based semiconductor laser having an emission wavelength in a 780-nm region is adopted. However, since such an optical pickup includes two packages of the AlGaInP-based semiconductor laser and the AlGaAs-based semiconductor laser, the problem of increase in size and, moreover, the problem of increase in size of DVD apparatus arise. To solve these problems, an integrated semiconductor light-emitting device including a plurality of types of semiconductor light-emitting element with different emission wavelengths having light-emitting structures made of semiconductor layers grown on the same substrate are proposed (in Japanese Unexamined Patent Publication No. 11-186651).
FIG. 9 illustrates a structure of the conventional integrated semiconductor light-emitting device described above. As illustrated in FIG. 9, in this conventional integrated semiconductor light-emitting device (laser device), an AlGaAs-based semiconductor laser LD1 with an emission wavelength in a 700-nm region (e.g., 780 nm) and an AlGaInP-based semiconductor laser LD2 with an emission wavelength in a 600-nm region (e.g., 650 nm) are integrated on an n-type GaAs substrate 201. These lasers LD1 and LD2 are separated from each other on the substrate 201. The n-type GaAs substrate 201 is, for example, a (100) orientated substrate or a substrate whose principal plane is oriented, for example, 5 to 15 degrees off from the (100) plane.
In the AlGaAs-based semiconductor laser LD1, an n-type GaAs buffer layer 211, an n-type AlGaAs cladding layer 212, an active layer 213 with a single quantum well (SQW) structure or a multiple quantum well (MQW) structure, a p-type AlGaAs cladding layer 214 and a p-type GaAs cap layer 215 are stacked on the n-type GaAs substrate 201. An upper portion (i.e., ridge portion) of the p-type AlGaAs cladding layer 214 and the p-type GaAs cap layer 215 form a stripe extending in one direction. An n-type GaAs current confinement layer 216 is provided on both sides of the stripe, thereby forming a current confinement structure. A p-side electrode 217 is provided on the stripe p-type GaAs cap layer 215 and the n-type GaAs current confinement layer 216 and is in Ohmic contact with the p-type GaAs cap layer 215. The p-side electrode 217 is, for example, a Ti/Pt/Au electrode.
In the AlGaInP-based semiconductor laser LD2, an n-type GaAs buffer layer 221, an n-type AlGaInP cladding layer 222, an active layer 223 with an SQW structure or an MQW structure, a p-type AlGaInP cladding layer 224, a p-type GaInP intermediate layer 225 and a p-type GaAs cap layer 226 are stacked on the n-type GaAs substrate 201. An upper portion (i.e., ridge portion) of the p-type AlGaInP cladding layer 224, the p-type GaInP intermediate layer 225 and the p-type GaAs cap layer 226 form a stripe extending in one direction. An n-type GaAs current confinement layer 227 is provided on both sides of the stripe, thereby forming a current confinement structure. A p-side electrode 228 is provided on the stripe p-type GaAs cap layer 226 and the n-type GaAs current confinement layer 227 and is in Ohmic contact with the p-type GaAs cap layer 226. The p-side electrode 228 is, for example, a Ti/Pt/Au electrode.
An n-side electrode 229 is provided on the back surface of the n-type GaAs substrate 201 and is in Ohmic contact with the n-type GaAs substrate 201. The n-side electrode 229 is, for example, an AuGe/Ni electrode or an In electrode.
In this conventional device, the p-side electrode 217 of the AlGaAs-based semiconductor laser LD1 and the p-side electrode 228 of the AlGaInP-based semiconductor laser LD2 are soldered to respective heat sinks H1 and H2 which are electrically separated from each other on a package base 300.
In the conventional integrated semiconductor laser device configured as described above, the AlGaAs-based semiconductor laser LD1 is driven by causing current to flow between the p-side electrode 217 and the n-side electrode 229, whereas the AlGaInP-based semiconductor laser LD2 is driven by causing current to flow between the p-side electrode 228 and the n-side electrode 229. Laser light with a wavelength in a 700-nm region (e.g., 780 nm) is obtained by driving the AlGaAs-based semiconductor laser LD1, whereas laser light with a wavelength in a 600-nm region (e.g., 650 nm) is obtained by driving the AlGaInP-based semiconductor laser LD2. Selection between driving of the AlGaAs-based semiconductor laser LD1 and driving the AlGaInP-based semiconductor laser LD2 is performed by, for example, operating an external switch.
As described above, in the conventional integrated semiconductor laser device, the AlGaAs-based semiconductor laser LD1 with an emission wavelength in a 700-nm region and the AlGaInP-based semiconductor laser LD2 with an emission wavelength in a 600-nm region allow laser light for DVDs and laser light for CDs and MDs to be obtained independently of each other. Accordingly, if this integrated semiconductor laser device is mounted as a laser light source on an optical pickup of DVD apparatus, playback or recording for one of DVDs, CDs and MDs is enabled. The laser structures of the AlGaAs-based semiconductor laser LD1 and the AlGaInP-based semiconductor laser LD2 are formed by semiconductor layers grown on the same n-type GaAs substrate 201, so that one package is sufficient for the integrated semiconductor laser device. As a result, the size of the optical pickup is reduced, thus miniaturizing DVD apparatus.
When a semiconductor laser is used as a light source of an optical disk system, laser light focused on an optical disk is reflected off the surface thereof and is fed back to the laser emission edge again. In this case, the optical disk serves as a complex resonator. The oscillation wavelength in a longitudinal mode determined depending on the complex resonator and the oscillation wavelength in a longitudinal mode determined depending on the resonator plane of the semiconductor laser itself differ from each other because these resonators have different optical path lengths. In addition, the effective end-facet reflectance due to complex resonator effects also varies. Accordingly, in one operation state, the oscillation threshold current value in the longitudinal mode determined depending on the complex resonator is smaller than that determined depending on the laser itself. As a result, the oscillation modes are replaced with each other in some cases. In such cases, reflected feedback light from the optical disk causes mode competition of the longitudinal modes, so that the optical output becomes unstable, resulting in occurrence of noise. This noise is referred to as optical feedback noise. When the level of this noise exceeds −120 dB/Hz in terms of relative intensity noise (RIN), actual operation is hindered.
To reduce the optical feedback noise in a semiconductor laser, multi-longitudinal-mode operation of the semiconductor laser is effective. The multi-longitudinal-mode operation is a laser oscillation state in which the wavelength (oscillation wavelength) at which laser oscillation occurs is not a single wavelength but is composed of a plurality of oscillation wavelengths. In the multi-longitudinal-mode operation of the semiconductor laser, mode competition is less likely to occur and excessive noise does not occur, so that a low-noise characteristic with a small influence of reflected feedback light from an optical disk is implemented.
To generate multimode oscillation, it is sufficient to perform pulse operation of a semiconductor laser. The pulse operation of the semiconductor laser is achieved by performing pulse driving of the laser using a high-frequency superimposing circuit. However, in this case, the problem of necessity of a new driving circuit and, in addition, the problem of a malfunction of another electronic device caused by a radio wave leaking from the high-frequency superimposing circuit arise.
To perform pulse driving of a semiconductor laser without a high-frequency superimposing circuit, it is effective to utilize a self-oscillation phenomenon in the semiconductor laser. In the self-oscillating laser, an absorption region formed in a waveguide to laser light is excited by the laser light itself, and the amount of light absorption decreases, resulting in that a saturable absorber to be transparent (absorption saturation) needs to be formed. When the absorber becomes transparent, the loss in the waveguide decreases and light power rapidly increases. When the light power increases, the number of carriers in the active layer consumed during induced emission increases and the carriers are rapidly lost, resulting in that a shortage of the number of carriers occurs and laser oscillation stops. In view of this, the light power of the self-oscillating laser performs pulse operation with time even under DC bias operation, thus obtaining multi-longitudinal-mode oscillation. In the semiconductor laser under self-oscillating operation, the carrier density in the active layer varies with time, and thus the refractive index of the active layer varies with time. The variation in refractive index of the active layer causes a variation of the emission wavelength, so that the line width of each oscillation spectrum increases. As a result, coherence with reflected feedback light from an optical disk is reduced.
As described above, a self-oscillating laser is important as a small low-noise light source capable of reducing excessive noise without using a high-frequency superimposing circuit, and application of this self-oscillating laser to the integrated semiconductor laser device (two-wavelength semiconductor laser device) described above is desired.